[llvm] [Support] Prefer AUX vector for page size (PR #126863)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-ppc64-aix` running on `aix-ppc64` while building `llvm` at step 3 "clean-build-dir".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/64/builds/2254

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 3 (clean-build-dir) failure: Delete failed. (failure) (timed out)
Step 5 (build-unified-tree) failure: build (failure)
...
56.297 [5138/10/150] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/DynamicLibrary.cpp.o
56.524 [5137/10/151] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Valgrind.cpp.o
56.752 [5136/10/152] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Watchdog.cpp.o
57.075 [5135/10/153] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/YAMLParser.cpp.o
57.273 [5134/10/154] Building C object lib/Support/BLAKE3/CMakeFiles/LLVMSupportBlake3.dir/blake3_dispatch.c.o
57.524 [5133/10/155] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Memory.cpp.o
57.596 [5132/10/156] Building C object lib/Support/BLAKE3/CMakeFiles/LLVMSupportBlake3.dir/blake3.c.o
57.734 [5131/10/157] Building C object lib/Support/BLAKE3/CMakeFiles/LLVMSupportBlake3.dir/blake3_neon.c.o
57.842 [5130/10/158] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/RWMutex.cpp.o
57.871 [5129/10/159] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Process.cpp.o
FAILED: lib/Support/CMakeFiles/LLVMSupport.dir/Process.cpp.o 
/usr/local/clang-17.0.2/bin/clang++ -DGTEST_HAS_RTTI=0 -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_LARGE_FILE_API -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/lib/Support -I/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/lib/Support -I/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/include -I/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/include -mcmodel=large -fPIC -Werror -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections  -O3 -DNDEBUG -std=c++17 -UNDEBUG  -fno-exceptions -funwind-tables -fno-rtti -MD -MT lib/Support/CMakeFiles/LLVMSupport.dir/Process.cpp.o -MF lib/Support/CMakeFiles/LLVMSupport.dir/Process.cpp.o.d -o lib/Support/CMakeFiles/LLVMSupport.dir/Process.cpp.o -c /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/lib/Support/Process.cpp
In file included from /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/lib/Support/Process.cpp:122:
/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/lib/Support/Unix/Process.inc:35:10: fatal error: 'sys/auxv.h' file not found
   35 | #include <sys/auxv.h>
      |          ^~~~~~~~~~~~
1 error generated.
58.003 [5129/9/160] Building C object lib/Support/BLAKE3/CMakeFiles/LLVMSupportBlake3.dir/blake3_portable.c.o
58.584 [5129/8/161] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Threading.cpp.o
59.512 [5129/7/162] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Program.cpp.o
60.788 [5129/6/163] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Signals.cpp.o
61.836 [5129/5/164] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Path.cpp.o
61.890 [5129/4/165] Building CXX object lib/TableGen/CMakeFiles/LLVMTableGen.dir/Error.cpp.o
62.225 [5129/3/166] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/VirtualFileSystem.cpp.o
63.093 [5129/2/167] Building CXX object lib/TableGen/CMakeFiles/LLVMTableGen.dir/DetailedRecordsBackend.cpp.o
64.039 [5129/1/168] Building CXX object lib/TableGen/CMakeFiles/LLVMTableGen.dir/JSONBackend.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/126863